I am interested in trying some ultra micro direct drive pager models. Further to my last thread on making spinners to protect the motor I have been making props.

Both Peter Frostick and Ralph Bradley suggested trying coke can ali to get a feel for current draws etc. This works well and they are quite easy to make and excellent for trying things out but not very suitable for the final flying prop. They tend to bend on impct, effecting balance and pitch.

I think I have found a nice way to make accurate and eventually effective micro props. My chosen material is 0.15mm epoxy glass board. I have chosen this because it is VERY springy, thin (and can be thinned further) and fairly available. Further to that it can be formed into curved sheets using heat.

The attached pic shows a 23mm diameter prop for a 4mm pager. A blank was first cut using scissors after a section of sheet was curved on the body or a soldering iron. The central section of said blank was then thinned a little in plan view with a round file. One of the blades was then sanded to shape with a 3M diamond sander (they are great BTW). The blank is cut in half and the blade used as a template for the other side. Working like this leads to almost identical blades.

These blades were then glued into a hub made from bamboo. The slots can be made easily with a rasor blade and the blades CA'd in place. Finally the blades can be sanded to a thinner section at the tip and even given a slight airfoil section if one wishes. Final colour can be added with a marker.

I think the next step is to make many different blades of differing shapes and sizes and find a way of producing lots of hubs. Then many pitches and diameters can be tried out to find a good match to the motors.

One idea I had was to use the toner transfer method from PCB etching to mark the epoxy glass board with the blade shapes. It is then easy to sand the blanks up to the pattern. This is quite accurate and that is important due to the high rotation speeds requiring good balancing etc. Of course CNC cutting is the other option but not needed.

I am looking for ideas for a method to make hub blanks and also on jigs for cutting the blade slots. Or perhaps an adjustable hub would be best, just glued when set correctly.

Prashanth, I was thinking of something where the blades might push on, have their pitch adjusted and then glued in place. Although I am sure it is possie to recreate the KP prop hubs at this size I suspect it is not worth it.

Here is another method, which I found quick and easy.
It is a simple balsa hub into which is glued a piece of plastic cable insulation selected to be a friction fit to the motor shaft. I push it onto a pin or piece of wire and turn it up against a fine grinding wheel until it is turned down into a hub or spinner.
The blades are yogurt pot sliced at -15degrees to get twist. They are scraped with the model knife to thin them and make a slight facimile of an airfoil section. The beauty of plastic blades is they bend on landing and do not dislodge the motor mount. At this size they seem plenty strong enough in operation.
The hub is given two cuts with a fine saw and the blades glued in with thick Ca glue.
The hub is pushed right up to the front bearing so an arrival can not move the shaft back in the motor.
This one pulls about 0.4amps from a 4.5 ohm 6mm pager, but does not seem to give anywhere near as much thrust as a geared motor and larger props of similar design and construction.

Images

Nice one! The idea of a plastic tube inside another of whatever material appeals. Plastic or indeed reinforced plastic in my case seem ideal materials. My blades can be bent back against the motor and don't break, they then spring back just as they were.

Don't worry about the static thrust, direct drive is not suited to creating static thrust, just check that the motor is not pulling too much and tweak the prop via flight testing. Ultimately you loose efficiency going direct drive I would have though but it depends on the model and what you are trying to acheive.

Nice props! Graham, I made some small props using little tubes with holes as hubs. I also used the insulation of some stripped wire ( The PVC sleeve ) as a hub on a prop. It's not too flexible at small sizes like <1cm. The sleeve is very thin, I think about 1mm dia or so. 2 thin CF rods stuck to the prop blades and the best part was that the CFR was a fairly tight fit into the sleeve, so I could adjust pitch and test. This worked very nicely for small loads, like a geared 4mm pager. If we could find thin stiff tubes like 1mm Outer Dia and .3-.4mm ID, we could use a .6mm drill bit for the motor shaft hole and use them as hubs, maybe thin capilary tubes?

cross drilled tubing is a good idea, i have very small brass tubing of about the right size. Of course if you do this you need to add short stubs to the blades quite accuratey. I can actually make slots in carbon etc down to about 0.17mm so that might be a really good way of doing it.

I am going to just have to bite the bullet and try and make something. Oh well back to the thesis for now.

OK, thesis can wait a few more hours. Just nipped into the lab and printed about 4 different prop profiles including a few CAM props scanned in from the web. I have made them in various sizes and trasferred them onto the epoxy glass, it was very easy indeed as this material sticks very well to the toner. You can also do it from the glass side and see the toner sticking, this also avoids toasting the paper that makes it harder to soak off. I'll post a picture later.

I basically have blades for three props of each of the 4 designs in 5 differnent sizes. Add an adjustable hub and we are in prop test heaven

Here are the blanks I made. A very simple process and no CNC in sight for a change. I'll post a pic of a trimmed and sanded blade in a while. It is very easy to be precise as the material is quite hard and the template rather obvious.

Images

While on that subject, this is an excellent way to make precise sheet parts of any type, the pixelito clones for example will require a number of precise and fairly identical parts, all you need is some cheap CAD and access to a laser printer, oh and an iron!

Picture 1: My prefered weapons. Engineers scissors, 3M diamond sanding pads (so good it hurts), flytying "hackle pliers", no I don't tie flies. They are excellent and really cheap, also good for holding tension in thread while trying to tie it.

Picture 2: Why the method is accurate, basically assuming you did a good job on the transfer and didn't mash the hell out of the toner then the result you get is as good as you can see. Trim with scissors then sand to match "perfectly". The more care you take the better, with a microscope you would be laughing.

Picture 3: Who shrank my CAM prop. A pair of finished blades waiting to be curved and glued into a hub. The toner could be left on but it is quite thick, It scrapes of easily enough but perhaps there is a chemical way to remove?